1. Field of the Invention
The present invention concerns a dry etching method used mainly for the production of semiconductor devices and, more in particular, it relates to a method and an apparatus for dry etching providing compatibility for anisotropic fabrication and high selectivity.
2. Description of the Related Art
In recent years, a demand for fine fabrication in super LSI has become severer and it is indispensable, for example, in etching treatment, a processing method of providing compatibility for fine fabrication at high accuracy with minimized dimensional conversion tolerance and high selectivity to underlying material.
However, in a case of plasma etching materials other than oxide films, an anisotropic shape has been ensured as is well-known by the presence of a so-called side wall protection film. The side wall protection film is formed by the deposition, on the side wall of a pattern of various deposits including organic polymers which are formed when reaction products formed during plasma etching are dissociated again in the plasmas and they serve to protect the side wall of the pattern and prevent the side wall from being etched.
By the way, since the side wall protection film is formed by the deposits from the reaction products, when a pattern formed by etching is convex and if the width of the pattern is fine, the thickness of the side wall protection wall is relatively increased excessively, making the width of the entire pattern larger than a desired width. In the same manner, when a pattern formed by etching is of a concave recess and if the width is narrow, the thickness of the side wall protection film is relatively increased excessively, making the entire pattern width narrower than a desired width. Accordingly, as various kinds of patterns have been made finer and the with of the pattern is made finer (narrowed) as described above, the dimensional accuracy of the obtained pattern is lowered when if it is intended to ensure the anisotropy of etching by utilizing the side wall protection film.
In order to overcome such a disadvantages, it has recently been attempted and attracted attention to apply etching while conducting exhaustion at high speed thereby ensuring a dimensional accuracy.
In the high speed exhaustion process, a pump of a higher exhaustion speed than that of existent etching apparatus is attached, and conductance of the etching gas is improved to, whereby the residence time of an etching gas during etching is shortened and dissociation of the reaction products in plasmas is suppressed during etching. According to the high speed exhaustion process, since the amount of deposits by redissociation of the reaction products can be decreased significantly, the absolute value for the dimensional conversion tolerance and variation thereof can be suppressed extremely effectively.
However, in the high speed exhaustion process described above, since the reaction products are exhausted rapidly, a supply source for the side wall protection film is decreased and the side wall protection film of a satisfactory thickness is not formed, the anisotropic shape can not be ensured enough, so that it results in an additional problem that the configurational accuracy of the pattern obtained upon applying overetching is worsened.
That is, if a bias applied to a substrate is lowered in order to ensure the selectivity relative to the underlying material, since the side wall protection film is thin and, accordingly, weak, occurrence of side etch or notching is inevitable. On the other hand, if the applied bias is increased in order to ensure the shape, selectivity relative to the underlying material is deteriorated.
As a technique capable of overcoming the problem that the selectivity and the shape are in a trade-off relation and capable of attaining both the selectivity and the anisotropic shape simultaneously, a so-called low temperature etching technique of cooling the wafer to a temperature lower than 0.degree. C. during etching has been proposed.
The low temperature etching technique has been disclosed, for example, as an invention made by K. Tsujimoto in Proceedings of Symposium of Dry Process (Oct. 24-25, 1988, Tokyo), p.p. 42-49.
In this technique, radical reaction is suppressed by lowering the specimen temperature, so that anisotropy can be ensured even under a low substrate bias.
However, even the low temperature etching technique has the following disadvantages.
At first, fabrication is difficult to a material such as W polyside in which vapor pressures of reaction products are different. This is because the vapor pressure of reaction products such as WCl.sub.x and WO.sub.x Cl.sub.y formed upon etching of WSi.sub.x is low, etching for WSi.sub.x can not be applied if the temperature of the specimen is lowered to such a temperature as convenient for etching the polysilicon.
Secondly, deltaT (difference between a temperature set for a specimen stage and a wafer temperature) is increased upon etching. That is, although temperature lowering is effective for ensuring the selectivity relative to the underlying Si, for example, in the fabrication of contact holes, since temperature lowering results a contact hole of a tapered shape due to deposition of an excessive polymer, so that setting for the low temperature condition is difficult as described above and, in addition, incident energy has to be increased inevitably in order to disconnect Si--O bonds in the fabrication of contact holes, which results in the increase of deltaT.
Accordingly, because of the disadvantage described above, even the low temperature etching can be applied actually only at a halfway temperature. In order to overcome such a disadvantage, it may be considered to vary the setting temperature for a specimen comprising, for example, a wafer between etchings for materials having different vapor pressures of reaction products or between just etching and overetching. However, the temperature can not be changed within a short time in an existent cooling system by a chiller using a liquid such as fluorinate as a coolant and, accordingly, existent low temperature etching can not be practiced at a temperature to provide a sufficient effect inherent to the low temperature etching technique as described above.